It is known that driving conditions vary due to different road surfaces and weather conditions. Roads may get wet, snowy and icy, and thereby the grip under the wheels of a vehicle changes. For the purpose of keeping traction and braking force at their maximum, torque applied to the wheels should be adapted to be as close as possible to the value depending on the weather conditions.
It is a known problem in wheeled vehicles that wheels of the vehicles may lose grip on the surface. Loss of grip may occur during acceleration of the vehicle or during deceleration of the vehicle. When the wheels are slipping during acceleration, the body speed of the vehicle does not increase together with the increasing rotational speed of the wheels. In some cases, control that prevents wheels from losing grip when accelerating is referred to as traction control.
In deceleration or braking of a vehicle, slip occurs when the wheels of the vehicle lose grip on the surface and rotate slower than the body speed. This may lead to a situation in which the wheels are locked and the vehicle still moves. In vehicles, control that prevents slip during deceleration is referred to as an ABS system (“Antiblockiersystem” or “anti-lock braking system”). This system effectively keeps the wheels in rotation during braking and maintains the grip between the wheel and the surface, enabling the steering of the vehicle even during heavy deceleration.
In electrically powered vehicles, each driving wheel may include its own electric motor and corresponding systems for driving the motor. The system for driving the motor may include an inverter, which produces an alternating voltage from a DC voltage source of the vehicle and supplies it to the motor. As is known, inverters can control electric motors precisely, and torque produced by the motor can be controlled.
In known electrically driven wheeled vehicles, preventing of slip is carried out by limiting the torque when slip is detected, and then returning the torque back to its original value. This may lead to a repetitive decrease and increase of torque, and the optimal accelerating or braking force may not be reached.